I found that the webkit browser will take the embed tag with a html src as a iframe tag, just like:
test.html:
<embed src="http://www.google.com"></embed>

Using the chrome browser to open the test.html, it shows that it was a iframe.

That would be a problem in some rich text application.

I know the type of embed tag may help, but there are so many types there, for example:
video/x-ms-asf(asf),video/mpeg (mpeg&#12289;mpg),video/x-msvideo(avi),videond./vrn-realvideo(rmvb),applicationnd./vrn-realmedia(rm),audio/x-pn-realaudio (ram&#12289;ra),audio/x-aiff (aif&#12289;aiff&#12289;aifc),audio/mpeg(mpga&#12289;mp3),audio/midi(mid&#12289;midi),audio/wav(wav),audio/x-ms-wma(wma),video/x-ms-wmv(wmv)

And sometimes we don't know the type of the url that supported by the users.